20 g of common salt is dissolved in 180 g of water. What is the mass percentage of the salt in the solution ?
- (a)5%
- (b)9%
- (c)10%
- (d)15%
Correct — C, 10%. Mass percentage of a solute is its mass divided by the mass of the whole solution, taken as a percentage. Here the solute is 20 g of common salt and the solvent is 180 g of water, so the solution weighs 20 + 180 = 200 g. That gives (20 ÷ 200) × 100 = 10%. The key's value therefore fixes the reading of the wording: the 180 g is the water alone, and the denominator is the 200 g solution, not the 180 g of solvent.
- (a)5% — Five per cent of a 200 g solution is only 10 g of salt, half of what was dissolved. It also comes out if the 200 g solution is mistakenly doubled to 400 g in the denominator.
- (b)9% — Roughly what you get by dividing 20 g by 220 g, that is by adding the salt to a 200 g quantity of water instead of to 180 g. The water in this problem is 180 g, so the solution is 200 g.
- (d)15% — Fifteen per cent of the 200 g solution would be 30 g of salt. No reading of the given figures produces it.
Concentration says how much solute sits in a stated amount of solution. Mass percentage, the simplest measure, is (mass of solute ÷ mass of solution) × 100, where the mass of the solution is the mass of solute plus the mass of solvent. Volume percentage is defined the same way for liquids in liquids. Both are ratios of the same kind of quantity, so the units cancel and the answer is a pure number — which is also a useful check that you have not mixed grams with millilitres.
The wording '20 g of common salt is dissolved in 180 g of water' has to be read carefully, because 'in 180 g of water' names the solvent, not the finished solution. If a student divides by 180 instead of 200 the result is about 11.1%, and the fact that no such option is printed is itself the signal that the intended denominator is the 200 g solution — which is the reading the official key's 10% confirms. Keep the two quantities straight and the arithmetic is a single line.
- Mass percentage of a solute = (mass of solute ÷ mass of solution) × 100.
- Mass of the solution = mass of solute + mass of solvent, so 20 g of salt in 180 g of water gives a 200 g solution.
- In this mixture salt is the solute and water is the solvent, since the solvent is the component present in larger amount.
- Mass percentage is a ratio of two masses, so it carries no units.
The salt goes over the 200 g solution, giving 10 per cent.
- Dividing by the mass of the solvent instead of the mass of the solution.
- Forgetting to add the solute's mass into the solution's mass.
- Mixing a mass with a volume — 180 g of water and 180 mL of water are numerically close only because water's density is about 1 g/mL, and that does not hold for other solvents.
NDA sets a one-line numerical on mass percentage, or asks which of solute and solvent is which in a named mixture.
No directly related past PYQ was found.
- practice — not a real PYQ
25 g of sugar is dissolved in 75 g of water. The mass percentage of sugar in the solution is
- (a)20%
- (b)25%
- (c)33.3%
- (d)75%
Answer(b) 25% — the solution weighs 100 g, so 25 ÷ 100 × 100 = 25.
- practice — not a real PYQ
In a solution of common salt in water, the solvent is
- (a)common salt
- (b)water
- (c)both salt and water
- (d)neither
Answer(b) water — the solvent is the component present in the larger amount, in which the solute dissolves.